1. Field of the Invention
Embodiments of the present invention generally relate to power converters and, more specifically, to systems and methods for testing worst case transients in a switching-mode power supply.
2. Description of the Related Art
Electric power is rarely used in electronic devices in the same form in which the power was originally produced or distributed. Therefore, a vast majority of electronic systems include some type of device that performs power conversions to convert power into a form that can be more readily used within those systems. One example of such a conversion device is a switching-mode power supply (SMPS). An SMPS incorporates a control circuit that switches transistors on and off in order to transfer electric energy from the source to the load through magnetic components and capacitors. A specific example of an SMPS is a switching DC-DC power supply that draws 100 Watts of power from a 48 Volt DC source and converts the 48 Volt DC input to a 3 Volt DC output, which can be used to drive a graphics card.
While in a steady-state, an SMPS provides a constant output voltage to a constant load. However, if the load suddenly changes (e.g., when a graphics card turns on or turns off), the output voltage may temporarily deviate from the constant value until the control circuit within the SMPS makes the adjustments necessary to return the output voltage to the desired constant value. Such temporary excursions from the desired output voltage are commonly referred to as “transients.” As is well-known, large transients in the output voltage of an SMPS may negatively impact the load. Therefore, before incorporating an SMPS into an electronic system, the SMPS is tested to determine the range of potential output voltages (referred to herein as a “transient swing”) that may be provided by the SMPS in response to varying loads. If the transient swing exceeds the limits that the load within the electronic system can withstand, then the power supply needs to be either modified appropriately or discarded.
A static load generator is oftentimes used to test a power supply to determine the range of output voltages the power supply will produce in the face of a varying load. For example, FIG. 1 illustrates a testing system 100 for testing the range of output voltages that a switching-mode power supply will produce in the face of a varying load, according to prior art. As shown, the testing system 100 includes a switching-mode power supply (SMPS) 110 connected to a static load generator (SLG) 160. The SMPS 110 includes a top field-effect transistor (FET) 112 and a bottom FET 114, controlled by a controller 120 though connections 122 and 124, respectively. The SLG 160 may be operated in two modes to provide a load for the SMPS 110. In one mode, the SLG 160 provides a constant load. In this case, by setting the duty cycle, frequency, and/or phase shift of the top FET 112 and the bottom FET 114, the controller 120 ensures that an input voltage (shown as “Vin”), is converted into the desired output voltage (shown as “Vout”). In the other mode, the SLG 160 provides varying loads and draws different amounts of power from the SMPS 110. The SLG 160 may be configured to change the loads either randomly or according to certain pre-programmed instructions. When the load provided by the SLG 160 changes, the output voltage provided by the SMPS 110 deviates from the desired value. In response to these changes in the output voltage, the controller 120 adjusts the duty cycle, frequency, and/or phase shift of the top FET 112 and the bottom FET 114 until the desired output voltage is once again established. Since such adjustments do not happen instantaneously, the transients above and below the desired output voltage can be detected and the range of potential output voltages produced by the SMPS 110 may be established.
One drawback to the above approach, however, is that the detected transients above or below the desired output voltage may not be the largest transients produced within the SMPS. The largest transients occur when the load provided to the power supply is either turned on or turned off at the same time the power supply controller turns off the top FET in the power supply. Such a state of the power supply is referred to herein as the “most vulnerable state,” and the transients that occur when the power supply is in the most vulnerable state are referred to herein as “worst case transients.” Thus, the occurrence of the worst case transients depends not only on the state of the load, but also on the state of the power supply. Since prior art static load generators, such as the SLG 160, do not take into consideration the state of the power supply, testing the worst case transients with a testing system such as the testing system 100 is ultimately unreliable.
As the foregoing illustrates, what is needed in the art is a technique for testing switching-mode power supplies that is capable of ensuring that the power supply output is tested when the worst case transients are present.